Conventionally, in a communication device such as a mobile phone, a filter device has been used to separate a transmission signal from another signal having different band, such as a reception signal, for example. Band-pass filters have been used for the filter device. These band-pass filters can be configured using a surface acoustic wave (SAW) resonator including one or more interdigital transducer (IDT) electrodes and reflectors, all of which are formed on a piezoelectric substrate made of lithium niobate or lithium tantalate. A filter configured with capacitive elements such as SAW resonators has a capacitive impedance, and therefore an inductive element is sometimes connected between the filter and an external terminal to achieve impedance matching.
FIG. 1A illustrates a first example of a conventional filter module 100 in which a matching inductor 110 is applied to a band-pass filter 120. As shown in the circuit diagram of FIG. 1A, the matching inductor 110 is connected in series with and preceding the filter 120. Referring to the Smith chart of FIG. 1B, the impedance of the first conventional filter module 100 in the passband of the filter 120 is indicated by a solid line 130. The impedance 130 appears on the upper half of the Smith chart and is inductive. For comparison purposes, a comparative impedance of the filter 120 alone or without the matching inductor 110 is shown by a dashed line 135. This comparative impedance 135 appears on the lower half of the Smith chart and is capacitive.
FIG. 2 illustrates a second example of a conventional filter module 100a in which a matching inductor 110 is applied to a band-pass filter 120. As shown in the diagram of FIG. 2A, the matching inductor 110 is connected in parallel with and preceding the filter 120. In other words, the inductor 110 is connected between the ground and a node 125 joining the input terminal 115 with the filter 120. Referring to the Smith chart of FIG. 2B, the impedance of the second conventional filter module 100a in the passband of the filter 120 is indicated by a solid line 140. The impedance 140 appears on the upper half of the Smith chart and is inductive. For comparison purposes, the impedance of the filter 120 alone or without the matching inductor 110 shown by a dashed line 145, which largely appears on the lower half of the Smith chart and is capacitive.